The present invention relates to methods and apparatus for printing signs, and also to a sign constructed in accordance with such method and/or such apparatus. The invention is particularly useful with respect to the production of multi-color signs which are front-illuminated in the daytime (e.g., by natural daylight) and back-illuminated at nighttime, such as used in billboards and the like, and is therefore described below particularly with respect to such an application. As will be described below, the invention could also be used in signs which are not back-illuminated, but which display the same or different pictures from its opposite sides.
Large signs used in billboards, particularly of the multi-color type, frequently include artificial back lighting at the inner face to enhance the image viewed from the outer face of the sign. However, such artificial back lighting tends to "wash out" the colors of the sign, that is, to make the colors appear much lighter than they really are because of the added artificial illumination. One method of minimizing such "wash out" effects, as described for example in U.S. Pat. No. 5,294,946, is to print the same image on both sides of the substrate, such that the double-sided image is provided with enough color density to decrease the tendency of the back lighting to wash out the colors. However, printing the image on both sides of the substrate limits the possible applications for such signs. For example, signs of this type are frequently pre-coated with an adhesive layer on one face of the substrate, covered by a protective layer which is removed, for adhesively fixing the sign to a mounting panel. However, where an image is printed on both faces of the substrate, one face would not be available for receiving the adhesive coating.
Nakazima U.S. Pat. No. 4,500,374 discloses a technique for producing illuminated signs including two printed layers separated by a semi-opaque layer. In the technique described therein, each ink layer is a printed layer of water-resistant ink printed on a pulp board sheet which sheets are bonded together and subsequently dissolved to leave only the printed layers. Such a technique is a gluing technique, and not an actual printing technique. Moreover, it requires special inks and substrates which would not dissolve with the pulp board sheets. In addition, such a technique does not permit volume low-cost production of signs of this type.